1. Field of the Invention
The invention relates to an adaptive system in a digital data receiver providing compensation for amplitude and phase distortions introduced by a data transmission channel.
2. Description of the Prior Art
It is known fact that at high transmission rates, a correct restitution, at the output of the transmission channel, of the signals applied to its input is no longer possible without the provision of a compensation circuit, designated equalizer, which is often formed by a non-recursive transversal filter, i.e. by a circuit which is capable of correcting the response of a transmission channel on the basis of a finite and weighted sum of partial responses available on the consecutive taps of an impedance network formed by delay lines. An equalizer of a conventional type having N weighting coefficients is shown in FIG. 1 (the article by J. G. Proakis and J. H. Miller, published on pages 484 to 497 in the periodical IEEE Transactions on Information Theory, Vol. IT-15, No. 4, July 1969 gives a description of such an equalizer having seven coefficients). Since the impulse response of the channel is not known and furthermore tends to evolve in the time, the equalizer must be adaptive, that is to say it must be capable of adjusting its weighting coefficients to the optimum values at the beginning of the transmission (this is commonly referred to as the acquisition or training phase of the equalizer), and of following thereafter any variations of the channel during the actual transmission phase.
Such an adjustment of the coefficients of the equalizer is generally obtained by iterative methods, such as the stochastic gradient method (actually, the circuits by means of which they can be implemented are relatively simple and are well suited to follow the slow variations of the transmission channel), but which inevitably meet the following obstacles: if the step used in the iterative method has too high a value the receiving system may become instable, while a small step size corresponds with a very slow convergence speed.
These two disadvantages can be avoided by using the direct resolution of the Wiener-Hopf equation, which gives theoretically the optimum coefficients of the adaptive equalizer (this resolution is described in the article "Non-iterative automatic equalization" published by P. Butler and A. Cantoni, June 1975, pages 621 to 633 of the periodical IEEE Transactions on Communications, Vol. COM-23, No. 6), but its complexity makes it actually very difficult to apply this direct method.